1. Field of the Invention
The embodiments of the invention generally related to reading the magnetic permeability of a material on or near a surface such as the reading or nondestructive testing of magnetic storage devices, and more particularly to devices and techniques used for probing the local magnetic permeability of magnetic materials.
2. Description of the Related Art
In the constantly evolving magnetic recording industry, information is generally stored as magnetic bits on thin ferromagnetic films. Generally, magnetic bits of information are stored in the direction of the magnetization in small, local regions of magnetic media (i.e., magnetic films). Magnetic films are typically used in a variety of devices that include magnetic random access memories (MRAM) and magnetic recording media. However, defects or magnetization reversals in these films can cause failure and potentially lead to serious accidents depending on the particular application implementing the ferromagnetic films.
Conventionally, in reading the magnetic bits of information, detection devices are used to measure the direction and amplitude of the magnetization of small regions along a magnetic track. Computer storage devices, such as magnetic disk drives utilize read/write heads to store and retrieve data. A write head stores data by utilizing magnetic flux to set the direction and amplitude of the magnetic moment of a particular area on a magnetic medium. The state of the magnetic moment is later read by a read head, which senses the magnetic fields.
Conventionally, read heads utilize giant magnetoresistance (GMR) read heads, which are spin valve transistors or other devices similar to spin valves. These GMR thin-film read heads employ a magnetoresistive structure, generally formed in a layered structure of ferromagnetic and non-ferromagnetic metals, to detect the magnetic moments of the data bits on the media. A sensing current is passed through the magnetoresistive material to detect changes in the resistance of the material induced by the data bits as they pass the read head. Spin valve transistors can be formed in different arrangements, but are usually configured as four-layer structures including a hard or pinned ferromagnetic layer, a layer that performs the pinning, a soft ferromagnetic layer, and a thin intervening conductor layer. The layer that performs the pinning is typically an antiferromagnet.
However, this stored magnetic information in the media can be destroyed (i.e., altered or deleted) by applying a large magnetic field that will modify the direction of the magnetization on the magnetic media. Because of this, there is a need for a more permanent technique of storing magnetic information in magnetic media. Moreover, there is a need for improved methods with heightened sensitivity for finding defects and micron size imperfections in the surfaces of materials.
Nondestructive testing of components during manufacture or in use is important to avoid malfunctions that can lead to lost time or injury. Many parts of magnetic storage devices are either ferromagnetic or contain designed or damaged openings on their surface that can be infiltrated with ferromagnetic particles. Thus, magnetic sensors can be used for nondestructive testing. Unfortunately, most conventional testing methods measure the magnetization and/or do not measure on a micron-size scale. For example, while magnetic atomic force microscopes (MAFMs) measure on a micron-size scale, they tend to measure the magnetic field rather than the magnetic permeability. Furthermore, MAFMs tend to be complicated, expensive devices that are very sensitive to vibrations. Measuring the magnetic permeability provides an additional method for nondestructive testing. For example, two components might have the same magnetization but very different magnetic permeability if one of them has been strained or work hardened. Thus, there is a need to measure the magnetic permeability of materials on a micron-size scale.